This invention relates generally to forming openings in substrates and more particularly to using such techniques to fabricate inkjet printheads.
Inkjet printheads are one of the many types of articles fabricated on silicon wafer substrates using photolithography techniques. A printhead is a drop-generating device having a plurality of nozzles or orifices through which drops of ink are selectively ejected. Ejection of an ink drop through a nozzle is accomplished using any suitable ejection mechanism, such as thermal bubble or piezoelectric pressure wave. One common architecture for a thermal inkjet printhead has a plurality of thin film resistors provided on a semiconductor substrate. An orifice plate is deposited over the thin film layer on the substrate. The orifice plate defines firing chambers about each of the resistors, a nozzle corresponding to each firing chamber, and an ink feed channel fluidly connected to each firing chamber. Ink is provided through an ink feed hole or slot formed in the substrate and flows through the ink feed channels to the firing chambers. Actuation of the resistor by a “fire signal” causes ink in the corresponding firing chamber to be heated and expelled through the corresponding nozzle.
Fabricating such inkjet printheads generally comprises forming an orifice plate on the frontside of a silicon wafer substrate and then forming an ink feed hole in the substrate. One known operation for forming ink feed holes comprises a hybrid laser micromachining and wet chemical etch slotting process. In this hybrid slotting process, a laser micromachining operation makes hardmask openings in a backside oxide layer and then laser micromachines blind trenches in the hardmask openings. The laser trenches must be machined to a specified depth, within a given margin. Next, a wet chemical etch process completes the ink feed holes by etching from both the backside and frontside to meet the final critical dimension (FCD).
While generally providing satisfactory results, this hybrid slotting process does experience occasional yield defects. One common yield defect seen with this process is the so-called “under-etch” defect in which insufficient etching occurs and the ink feed hole fails to meet its final critical dimension (FCD). A major contributor to the under-etch defect is poor frontside etching in the center region of the ink feed hole. Because the frontside etching occurs in the substantially closed chambers formed by the orifice plate, the hydrogen produced by the chemical reaction does not have space to escape and therefore impedes the etching process. Thus, frontside etch only initiates and etches along the edges of the ink feed hole, and the center region experiences minimal etching. As a result, it takes longer for the frontside and backside etches to meet and break through, thereby resulting in more under-etch defects.
Another common yield defect seen with this hybrid process is “laser punch-through” of the orifice plate. That is, breaking through the frontside of the substrate while laser micromachining the backside trench and damaging the orifice plate. The major contributor to laser punch-through of the orifice plate is the laser trench depth being targeted too deep and with small margin. In other words, to achieve desired etching, the backside trench is machined very deep, and thus very close to the frontside of the substrate, which can result in occasional punch through to the orifice plate.